Apparatus for xerographic development



July 14, 1959 E. P. MATVTHEWS 2,894,486

- APPARATUS FOR XEROGRAFHIC DEVELOPMENT Original Filed Feb. 18, 1955 2 Sheets-Sheet 1 SENSITIZATION (CHARG' N6) f EXPOSURE DEVELOPMENT TRANSFER AND/OR Flxme POWDER C. LOUD GENERATOR INVENTOR. EARNEST PAUL MATTHEWS AWORWEY July 14, 1959 E. P. MATTHEWS 2,894,486

APPARATUS FOR XEROGRAFHIC DEVELOPMENT Original Filed Feb. 18, 1955 2 Sheets-Sheet 2 PowDErz POWDER I 50 c LOUD c LOU D 4,5 1g 5 GENERATOR c HAIZGI N6 INVENTOR.

EARNEST PAUL MATTHEWS ATTORNEY United States Patent APPARATUS FOR XEROGRAPHIC DEVELOPMENT Earnest Paul Matthews, Alliance, Ohio, assignor, by

mesne assignments, to Haloid Xerox Inc., a corporation of New York Original application February 18, 1955, Serial No. 489,083. Divided and this application February 13, 1957, Serial No. 639,954

3 Claims. (Cl. 118637) This invention relates to an apparatus for the development of electrostatic latent images.

In Xerography it is usual to form an electrostatic latent image on a surface. One method of doing this is to charge a photoconductive, insulating surface and then dissipate the charge selectively by exposure to a pattern of activating radiation. Other means of forming electrostatic latent images are set forth in US. 2,647,464 to James l Ebert. Whether formed by these means or any other, the resulting electrostatic charge pattern is conventionally utilized by the deposition of an electroscopic material thereon through electrostatic attraction whereby there is formed a visible image of electroscopic particles corresponding to the electrostatic latent image. Alternatively, the electrostatic charge pattern may be transferred to an insulating film and the electroscopic particles deposited thereon to form the visible image. In any case, this visible image, in turn, may be transferred to a second surface to form a Xerographic print.

The process of depositing the electroscopic powder on the electrostatic image to render the electrostatic image visible is called the development step and is one of the most critical steps of the entire process. The step is of particular importance both on machines designed for continuous operation with any type of copy and in processing continuous-tone images. Now, in accordance with the present invention, an appaartus is provided for substantially improved development of an electrostatic latent image.

Fig. l of the attached drawings is a block diagram showing the position of the development step in an overall Xerographic process which results in a visible image.

Fig. 2 is a diagrammatic side elevation in cross section of apparatus according to one embodiment of the invention showing the initial step in development using this embodiment.

Fig. 3 is a diagrammatic side elevation in cross section of the same embodiment showing the second step in development.

Fig. 4 is an isometric drawing of the same embodiment with a cutaway section.

Fig. 5 is a diagrammatic side elevation in cross section of apparatus according to another embodiment of the invention.

Fig. 6 is an isometric drawing of the embodiment shown in Fig. 5 with a cutaway section.

As shown in Fig. 1, the general Xerographic process involves the formation of an electrostatic latent image. This is generally, although not always, preceded by a treatment to sensitize the surface on which the electrostatic image is to be formed. The electrostatic latent image to be useful must then be rendered visible, which is done in a development step. This is accomplished by depositing electroscopic particles either on the surface on which the image Was formed or on an insulating surface to which the electrostatic latent image has been transierred.

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It is evident that no picture can be better than its development step permits. About the coarsest type of image reproduced by a Xerographic process requires a resolution of at least about 50 lines per inch. Commercial, line-copying machines generally have a resolution power of about to 250 lines per inch. The process used in obtaining this resolution is set forth in US. 2,618,552 and involves the use of a finely-divided, colored material called a toner deposited on a slightly more coarsely-divided material called a carrier. This twocomponent developer is cascaded across the electrostatic latent image areas. The control of the concentration of toner in the two-component developer becomes extremely difiicult in the continuous operation of such linecopy machines. When applied to continuous-tone development where resolutions of about 50 or more lines per millimeter are often desired, it has been found impossible to obtain this high quality of reproduction using such a system. Accordingly, a system known as powder cloud development is preferred. This is the system incorporated in 1.

As shown in Fig. 1, a powder cloud is generated. The cloud so produced is then charged, the charging step constituting either a separate step or utilizing the inherent charge on the cloud in the case of a cloud of dry particles. A cloud so generated and charged is then contacted with the electrostatic latent image in the development step. The visible image so produced may be used as such, permanently aflixed to the plate, or may be transferred to another material as a sheet of paper or plastic, as is well-known to those skilled in the Xerographic art.

The instant invention relates to a process and apparatus therefor which represent a substantial improve ment in the art of powder cloud development. Many of the methods of powder cloud development used heretofore have been characterized by various diificulties, thus: When the powder cloud in its flow travels over an area that should be white and then over an area that should be dark, the leading edge of the dark area is not developed. On the developed image the dark area appears to be torn so that a white background shows through. Because of this appearance the flaw is referred to as tearing. Another difiiculty occurs when the powder cloud travels over a large, dark area and then over a large area that should remain white. Although the white area is void of charge, it does not remain free of powder. Developer particles deposit in streaks through this area in amounts roughly proportional to the length of the dark area that immediately precedes the white area. This difiiculty is termed streaking. Still another difficulty is that developed images are not uniformly developed end-to-end nor does extending the development time remedy this defect as, apparently, part of the charge in the image areas at one end of the plate are erased in the development pro ess. The process of the instant invention represents substantial improvement on all these. in general, the present invention accomplishes these objectives by reversing the flow of the powder cloud over the image areas during the development step. The process of the invention and apparatus for its accomplishment will now be set forth in more detail.

According to the present invention, a cloud of electroscopic particles is generated in a suitable cloud-forming device; for example, as by agitation of a powder mass in a closed container with a rotating brush, as in U.S. 2,357,809 to C. F. Carlson, or a rotating air mass as caused by laterally-located nozzles. If a cloud of liquid droplets is desired, it may be produced by any means known to those skilled in the art as by spraying through an atomizer. The cloud by whatever means produced is then charged. Where a cloud of dry particles is used, any method of generating the cloud will almost necessarily produce a charge thereon. For some purposes, the chargeso produced will .be adequate. If his desired to produce .a more uniform charge, however, .other .devices, such .as trihoelectric charging (as by passing the electroscopic powder through .a narrow tube .of suitable material in turbulent flow), .corona charging or other charging device, may be used. In the case of a cloud of liquid droplets, charging means such as induction charging, corona charging, and so forth, may be used. The charged cloud is then optionally passed into a collecting zone and then through suitable valving means into the development zone alternately from each end.

The development zone is the .area between the development electrode and the image-bearing surface. The development electrode .draws the field of force of the image externally above the image-bearing surface. To do this, the development electrode should be no further from the image-bearing surface than A and, desirably, should be no further than A". For high quality resolution, the spacing should be no more than 4 It should, of course, extend over the entire image surface. The charged electroscopic particles are passed over the imagehearing surface within the field of force. Passing the powder cloud once from each end through the development zone is suflicient to realize the advantages of the instant process. If desired, however, the direction of flow may be alternated several times in the course of development with some improvement resulting from the greater number.

The general nature of the process of the invention having been set forth, specific embodiments of the in vention will now be described. The invention, however, is not limited to these embodiments which are presented necessarily for purposes of illustrating means of Working the instant invention.

.As shown in Fig. 2, the apparatus illustrated contemplates a support plate having entrance holes 13 and 14 and exit holes 11 and 12. Superimposed on the support plate in movable relation thereto is a base plate 15 also having entrance holes 18 and 19 and exit holes 16 and 17. Positioned thereon are spacing means 20 defining left and right chambers 21 and 22. On top of the spacer is the development electrode 23 having slots therein, 24 and 25, which may serve as either entrance or exit slots. Positioned closely adjacent to the development electrode by means of spacers 26 is an electrostatic image-bearing member comprising a photo-conductive insulating layer 29 coated on a conductive backing 27. Stop plate 28 aligns the assembly.

In operation, as shown in Fig. 2, which depicts the first step of development, the powder cloud flows through entrance slots 13 and 18 into right chamber 22 where it expands and then flows through slot 24 into the development zone 48 between the development electrode and the image-bearing member. Passing through this zone, it leaves by means of slot where the cloud passes into chamber 21 and then through the exit slots 17 and 12. This position is maintained for no more than one-half the development cycle.

Then the assembly is shifted as shown in Figure 3. This may be done by holding the support plate stationary and moving the rest of the assembly (as was done here), or by moving the support plate (which requires flexible tubing to carry the powder cloud to and from the apparatus) and holding the rest of the assembly stationary. In this position the powder cloud now enters through entrance holes 14 and 19 into chamber 21 where the cloud fans out, flows through slot 25 into development zone 48, leaves the development zone through slot 24, enters chamber 22, and leaves through exit holes .16 and 11. cycle may be repeated as often as desired in .the course of the development of the electrostatic image. BY. this means the direction of flow'of the powder cloud 4 with respect to the xerographic plate being developed may be reversed as often as desired in the course of development.

In this embodiment of apparatus according to the invention the left and right chambers 21 and 22 afford additional advantages beyond those resulting from the process of alternate development from each .end of the development zone. Thus these chambers provide a smoother and more uniform powder cloud as it is introduced into the development zone with resultant improvement in print quality. In addition, the chambers in combination with the slot exhaust promotes a more uniform development.

Fig. 4 shows an isometric view of the same development apparatus.

Fig. 5 shows another embodiment of the invention wherein valving means are substituted for physical move ment of surfaces.

In the apparatus shown, the powder cloud is generated 42, charged 46 and fed to a two-way'valve 31 by a tube 43. The valve diverts the powder cloud to one of the two exit tubes, say, the right-hand tube 30. The powder cloud flows through this tube to the entrance .slot 44 where the air pressure exerted by the cloud forces up gate means 32 thereby permitting the powder cloud to flow through the development zone 33 in roughly laminar flow leaving through exit means 34. When no more than half the'development time has elapsed the valve position is changed sending the powder cloud through the left-hand tube 35 thence to the entrance slot 45 where, by means of the pressure exerted by the powder cloud, gate means 36 are forced up permitting the powder cloud to flow through the development zone in approximately laminar flow leaving thnough exit means 37. In the apparatus shown, 38 is the development electrode and 39 the xerographic plate having an image-bearing surface 40. Spacing means 41 are provided to position the xerographic plate at the correct distance from the development electrode, namely, no more than about A apart.

Fig. 6 shows an isometric view of the same development apparatus.

On whatever type of machine the process ,of the ;invention is used, a preferred method of operation is to use alternate development periods of short duration. For example, if in a total development time of 4 seconds the image is developed for 2 seconds in each direction, tearing and streaking will be definitely reduced. However, if development were for periods of /2 second from each direction alternately, a much more complete covering of tears and streaks would be achieved. Hence, for any given overall development period it is preferred to reverse the direction of powder cloud flow through the development zone almost as frequently as the equipment permits within the available time.

Where the electrostatic image is borne by a photoconductive insulating layer coated on a conductive backing as in the embodiments shown in Figs. 2, 3 and S, desirably means are provided for applying an electric potential difference between the conductive backing member 27 for the photoconductive insulating layer 29 and the development electrode 23. These means may include, for example, as shown in Fig. 2, a battery 46 or similar DC. power source connected through a'vpotentiometer 47 to, in this case, the conductive backing 27. The conductive backing and photoconductive insulating layer are, of course, insulated from the development electrode in such a system and the electrode itself is grounded as shown. If desired, the potential may be applied to the development electrode and the conductive backing member grounded.

Generally, the powder cloud is charged with a charge of polarity opposite to the polarity of the electrostatic image. However, this is not necessarily so. Thus, reversal development may be eflfected by charging the zpowder cloud with a charge having the same polarity as the image areas. Hence, the powder is repelled from the image. Reversal development may also be obtained by use of the proper bias from potentiometer 47 between the development electrode 23 and the backing member 27. In this system, the potentiometer is adjusted to bias the conductive backing member relative to the development electrode to the highest voltage on the photoconductive insulating layer 23 and of the opposite polarity to the electrostatic image. Where the potentiometer is connected to the electrode and the conductive backing member is grounded, the potential is of the same polarity as the electrostatic image. In either case, the field applied opposes the image field. The powder (charged to a polarity opposite to that to which the photoconductive insulating layer was initially charged) is attracted to the uncharged background areas to develop a reversal print.

Other modifications and variation of the basic invention will, of course, be obvious to those skilled in the art. Whatever variation of the basic process is used, however, it is possible by this process to substantially reduce tearing and streaking while greatly improving end-toend uniformity of the developed xerographic image.

This application is a division of my copending application Serial Number 489,083, filed February 18, 1955.

I claim:

1. Apparatus for developing an electrostatic latent image comprising a powder cloud generator, means for changing cloud particles, a development electrode, support means to receive and support an electrostatic imagebearing member adjacent to one side of the electrode and closely spaced therefrom thereby defining a development zone, a base plate on the other side of said electrode and positioned close thereto by a spacer which forms right and left chambers between the said base plate and the said electrode, a slot extending from said right chamber through the electrode to said development zone, and a slot extending from said left chamber through said electrode to said development zone, exit slots in said base plate at the opposite ends of both right and left chambers, entrance apertures extending through said base plate into both the right and the left chambers, said entrance apertures being positioned between the exit slots in each chamber and the spacer dividing the space between the electrode and the base plate into right and left chambers, stop plates at each end of the assembly to align the plates, a support plate positioned adjacent to the side of the base plate opposite from that presented to the electrode and positioned in movable relationship thereto, exit slots at each end of said support plate and extending therethrough, entrance apertures extending through said support plate intermediate the exit slots and positioned on each side of the center of the said support plate, said exit slots and entrance apertures in said support plate being so positioned that when said support plate is abutting on the stop plate on either side of the said assembly only one exit slot and the entrance apertures on the side of the center opposite from that exit slot are aligned with the corresponding slot and aperture in the said base plate, and means to feed the charged cloud particles into said entrance apertures.

2. Apparatus for developing an electrostatic latent image comprising a powder cloud generator, means for charging cloud particles, a development electrode, support means to receive and support an electrostatic imagebearing member adjacent to the electrode and closely spaced therefrom thereby defining a development zone therebetween, exhaust means at opposite ends of said electrode, walls defining two chambers, each chamber communicating with said development zone through a narrow aperture through said walls positioned interiorly from each of said exhaust means, pressure responsive gate means in each narrow aperture, means connecting each narrow aperture to a 2-way valve, and means to feed the charged cloud particles into said valve.

3. Developing mechanism for electrostatic images comprising in combination a development electrode, means for positioning an electrostatic image-bearing member in uniform closely spaced relationship with said electrode to form a developing zone therebetween, walls defining two entrance chambers, each chamber communicating with said development zone through a narrow aperture through said walls at opposite ends of said electrode, means for producing and blowing a gas suspension of finely-divided electrostatically charged material, conduit means to feed said gas suspension to said entrance chambers and valve means in said conduit whereby said gas suspension is fed into each entrance chamber alternately.

References Cited in the file of this patent UNITED STATES PATENTS 2,358,138 Blanchard et a1. Sept. 12, 1944 2,614,901 Jacob Oct. 21, 1952 2,711,481 Phillips June 21, 1955 2,725,304 Landrigan et al. Nov. 29, 1955 

